This invention relates to a sensor to be used in medicine and more particularly to a sensor for providing biofeedback to help patients relearn physical function, or prevent complications that impede their physical function.
There are currently no such devices in general use by patients either in acute care, rehabilitation or at home in spite of the need for devices of this nature. However, the need for them is compelling.
For instance, biofeedback may be provided for monitoring the weight applied to a limb ("limb load monitoring"). A major example relates to limitation of weight bearing after leg fracture or hip replacement. Total hip replacements alone are performed 200,000 times per year in the United States. The surgeon prescribes "toe-touch" or "partial" weight bearing almost universally. Partial weight bearing continues until healing has progressed sufficiently to allow full weight bearing safely. In almost all rehabilitation centers there is no way to assure compliance with these orders except by observation. Due to the absence of suitable devices, the true cost of excessive weight bearing has never been systematically studied. However, it is reasonable to hypothesize that un-monitored weight bearing prolongs healing time, duration of hospitalization, and dependent status. Limb load monitoring could significantly reduce this apparent loss of time and money.
Additionally, there are applications to gait training. Limb load devices would prompt amputees to symmetrically bear weight on prosthetic limbs. (This is a prelude to progressive ambulation). Children with cerebral palsy could be taught to put weight on the heel first on planting the swing phase leg during gait. Time on an inpatient rehabilitation service could be reduced, and safe ambulation promoted.
Other potential applications include rewarding the patient for meeting strength goals, e.g., isometric grip strength and knee strength. Further, individuals with rheumatic diseases could greatly benefit from effective limb load monitoring.
These devices, if may generally available, could be used to teach the safe use of wheelchairs to prevent costly falls, or proper pressure relief methods to reduce incidence of gluteal decubiti. Further consideration also must be given to take into account the cost to hospitals of potential malpractice litigation involving medical complications. Overall, the cost saving due to reduced morbidity and length-of-hospital stay could be of great financial significance.
The remainder of this background outlines the prior art of rehabilitation force transduction. Capacitive, resistive, hydraulic, pneumatic and miscellaneous transducer types have all been proposed in the past. However, each has shortcomings that may explain their clinical scarcity and none are widely used by patients.
1. Capacitive Devices
Capacitive transducers have inherent benefits in monitoring loading on limbs or other body parts. They provide a relatively linear increase of capacitance with applied weight. (Therefore, one may use simple, low cost electronical circuitry). They measure total weight or force between complex, irregular surfaces. However, they have been ignored over most of the past decade by those skilled in the art in favor of resistive devices. This has been due to the inability of anyone in the field to find an acceptable dielectric.
The following paragraphs lists concepts and definitions important in evaluation of capacitive transducers.
The dielectric is the insulating middle layer between conductive plates of the sensor. Mechanistically, force applied compresses the dielectric, bringing plates closer together and increasing capacitance.
The effectiveness of such devices is dependent on the properties of the dielectric materials. Creep refers to the long term change of dielectric thickness after initial loading. Hysteresis compares the difference in compression (i.e., capacitance) at a given load with the load increasing, and the load decreasing. Sensitivity is the ratio of capacitance in the loaded and unloaded state, per application of unit weight. Dynamic response is the speed with which the sensor achieves steady state after weight is applied. Resiliency is the ability of the dielectric to retain its unloaded thickness (and baseline capacitance) after a long duration of static or dynamic loading. (A constant application of weight is static, and a ballistic application, dynamic). All these properties are a function of materials composition and structure (e.g., whether a foam or a solid). The ideal dielectric has zero hysteresis and creep. It also has very high resilience, sensitivity, and dynamic response.
The best known published prior art on capacitive sensors is the Krusen Limb Load Monitor. (Craik, R. and Wannstedt, G., Proceedings 2nd Conf. Devices and Systems for the Disabled, 1975, 19-24). The transducer consists of three layers of copper--Mylar(R) laminate separated by two layers of "closed cell foam tape". The whole device has the shape of an inner-sole. The characteristics of the dielectric, however, were not published. The device is manufactured by Electronics Quantification Inc., Plymouth Meeting, Pa. Disadvantages of the device include lack of adaptability to orthotics (i.e., braces), assistive devices or wheelchairs. Further, it is not designed for upper extremity use, and has a high cost ($600.00 per item).
Additionally, conventional polyethylene foam tape (the most likely dielectric material considering the limited public disclosure of the device) has significant hysteresis and creep, and also tends to "bottom out" with repeated compression. One further expects that the sensors need to be frequently replaced. Perhaps for these reasons the device is little used by the rehabilitation community after 15 years on the market. It is not well known even among rehabilitation engineers. Considering the size of the potential market, the device has been a failure.
After examination of this device, Miyazaki and Ishida published data on an apparent improvement. The dielectric is in this case two layers of Neoprene sponge, laminated between three layers of copper foil. (Miyazake, S., and Ishida A., Medical and Biological Engineering and Computing, 1984, pp. 309-316). However, Neoprene, like polyethylene, has limited resilience to static and dynamic loading. It also tends to "bottom out" with repeated use.
On initial testing there was good agreement between force plate and transducer data on patients with foot shapes that were not markedly abnormal. However, higher local pressures from deformed feet caused excessive error (i.e., overestimation of weight). Stiff plastic was therefore built into the laminate to spread out area of force application. As a result, the device was cumbersome, thick and stiff.
As published, it cannot be adapted to shoes, orthotics, assist devices or wheelchairs. No subsequent work using this device has been published. Apparently, it has been abandoned.
2. Resistive Devices
A second major category of sensors for rehabilitation are resistive devices. All of the recent public disclosure and most of the patented prior art is of this group.
Force Sensitive Resistors (FSR's) are marketed by Interlink Corp. They are thin (0.25 mm thick), physically flexible,-inexpensive devices available in 1 cm.sup.2 squares. Externally there are two adjacent Mylar (R) sheets. On one Mylar layer is a high resistance polymer or carbon film, and on the other is an interdigitated pattern of open ended conductors. Ruggedness appears to be good with extended use. It may be used with simple electronical circuitry.
For these reasons FSR's have been proposed for "everyday" rehabilitation pressure measurement in several applications. These include monitoring pressure under insensate feet in patients with diabetes and leprosy (M. Maalej, et. al., IEEE, 9th Conference of the Engineering in Medicine and Biology Society, 1987, pp. 1824, 1824). A finger and hand exercise device and "spot" high pressure sensor for decubitus prevention have also been described (Hyman, W. et. al., RESNA, 13th annual Conference, 1990, pp.201, 202). It additionally has been proposed for motorized prosthesis control (Heckathorne, C., RESNA, 12th annual Conference, 1989, pp. 224-225).
However, there are several significant drawbacks of these devices. The most significant is non-linearity; response is substantially exponential. Unloaded resistance is effectively infinite, and resistance falls rapidly as 10-20 pounds of weight is applied. Above this "breakpoint", resistance approaches a minimum asymptotically. Above 50 lbs resistance change may be obscured by noise. In terms of noise, the device demonstrates significant hysteresis that varies from device to device and trial to trial with the same device.
For processing, the response of each sensor must be linearized. Usually this is done by a microprocessor. However, it may be done (less successfully) with simple integrated circuits. To find the weight applied to a large irregular contour, the outputs of many linearized sensors must be summed. Using more sensors makes accuracy less dependent on variations of contour. However, this also increases complexity of necessary electronics, reduces reliability and dramatically increases current drain (i.e., decreases battery life in a portable device). Because of battery drain alone, FSR's would be unacceptable to use in a wheelchair embodiment.
To use a minimum number of sensors, FSR's must be placed at points of maximum pressure. For instance, for limb load monitoring sensors may be placed on the first and fifth metatarsals and the heel. However, many patients have irregular foot contours or may wear a cast. For them, sensor placement must be customized. Biofeedback for hand grip strength using FSR's presents an even more serious problem, as hand size and placement are difficult to control. Thus, FSR's lack flexibility where total pressure is to be obtained over complex surfaces. In contrast, capacitive devices are strong in this category, as they are in terms of current drain. Indeed, FSR's and other resistive devices are most reliably employed where local pressure are to be mapped and quantified over two dimensions, e.g. to prognosticate skin areas at risk for breakdown and ulceration. Calculated forecasts from resistive devices, readouts, may guide the prescription of custom shoes.
One such device is based on the flexible force sensor developed by Polchaninoff in 1984 (U.S. Pat. No. 4,426,884). Its electrical behavior (and disadvantages) is similar to the FSR. Twenty of these sensors are incorporated into the "multi-event notification system for monitoring critical pressure points on persons with diminished sensation of the feet" shown by Goforth in his 1987 patent (U.S. Pat. No. 4,647,918). Resistors are placed under the heel, lateral plantar surface, metatarsal heads, and toes. The information from each is transmitted to a microprocessor on the belt. However, drawbacks of complexity, cost, and fragility argue against the use of this device for limb loading after orthopedic procedure or for gait training. Indeed, many appropriate patients at risk for foot ulceration either cannot afford this device or cannot learn to operate it.
Tecscan Inc. (Boston, Mass.) has developed a pressure mapping system for gait analysis (Sensors, May 1991, pp. 21-25). Not 20 but 960 resistive elements are continuously sampled by a personal computer during ambulation. Graphics of excessive local pressure on the foot are displayed in real time. A similar device has been developed by Tecscan for determining gluteal pressure contours, and may be used for evaluating seating systems. Drawbacks of both are similar to the Polchaninoff device (i.e., high cost, lack of durability, fragility, complexity). The Tecscan limb load device is very expensive, exclusive of the cost of the required personal computer. The seating evaluator is at least this expensive.
3. Hydraulic Devices
Hydraulic devices easily determine total weight between irregular surfaces. They are similar in this respect to capacitive devices. However, by definition they do this by means of fluid pressure in a closed chamber. Thus, they tend to be large, heavy, bulky, thick or rigid. Hydraulic or (conceptually similar) pneumatic systems have been applied to limb load detection, grip strength detection, and seating pressure systems.
Two patented devices are designed to provide objective immediate feedback of limb loading (Sipe, U.S. Pat. No. 3,974,491; Pfeiffer, U.S. Pat. No. and 3,791,375). Both employ reservoirs below the heel and sole. Weight compresses these reservoirs and is monitored by a transducer on the ankle. There are several important disadvantages, including the need for a dedicated shoe. Also, the transducers are difficult to put on and take off, and the heights of these devices may impede ambulation in an individual whose walking is already impaired and possibly unsafe.
Hydraulic devices for measurement of grip strength are known as dynamometers. The devices are large and heavy. Force is read off a pressure gauge, which is difficult for the patient to read. Due to these limitations, they are not designed for use during exercise. They are certainly inappropriate for the elderly or frail arthritic who would most benefit from grip strengthening.
Pneumatic devices have been developed for measurement of pressure beneath bony prominences in evaluation of wheelchair seats. However, they are essentially flattened, modified balloons. Due to size, shape and fragility they are not appropriate to permanently monitor weight on wheelchairs. They are not adaptable to the wide variety of wheelchairs and wheelchair cushions now available.
4. Miscellaneous Devices
A mechanical switching device, patent granted to Gradisar, (Gradisar, U.S. Pat. No. 3,702,999) measures limb loading. Each of two switches has two metal discs separated by an "O" ring. As weight is applied to the top disk the "O" ring is compressed until contact is made, tripping a buzzer. By adjusting the set screw on one disk, the weight at which the buzzer sounds may be adjusted. This device is subject to mechanical problems and is difficult to adjust to the "trigger" weight, and only one type of shoe (a "cast boot") may be used.
Unpatented art that has been applied to biomedical force measurement include strain gauge and piezoelectric technologies. Strain gauges are made of coils of thin high-resistance wire or metal or silicon impressions on a substrate. Because of their high level of accuracy, these transducers are good for gait research. However, they are fragile, expensive and consume large amounts of power. Therefore they are unsuited for the demands of daily therapeutic use. Additionally, the signal from them is low, requiring extensive amplification. Thus, the complete system demands excessive space, a controlled environment and a large budget.
5. Summary
In summary, the prior art intended for weight bearing biofeedback has not gained commercial acceptance due to disadvantages of complexity, high cost, poor reliability, durability and concerns about safety. Force sensitive resistors are promising for certain limited applications. However, FSR's are substantially non-linear. Dedicated electronics must linearize and integrate outputs to measure weight. For large, irregular surfaces many FSR's must be used. This increases complexity, cost, and current drain. It decreases durability and battery life.
However, capacitive transducers inherently linearize and integrate force information. Capacitive transduction is independent of body contours and transducer contour. Devices need not be customized; they are readily interchanged between patients. They are simple, inexpensive, reliable, portable and have the best expected battery life.
One would think capacitive transducers are the best candidate for an "all-purpose" clinical weight bearing device discussed at the outset. However, capacitive transduction has apparently been abandoned by those skilled in the art due to non-existence of a suitable dielectric.